Switcher Driving Motor for Speed Control...

[Ricketty C]

We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load, so a PWM drive at a high enough frequency would approximate a constant current to the motor. A smoothing cap at the source (we already have 2000 uF) helps that a lot. The H-bridge would be retained to allow the polarity to the motor to be reversed.

Is this a method used? It would lower the I2R heating in the motor with a lower continuous current rather than a higher pulsed current with a duty cycle.

We have scope plots showing a 500 Hz pulse rate is not fast enough to make that work. I need to ask if we can get some better measurements to see how long it takes for the motor current to drop significantly when the H-bridge removes the power connection and the reverse diode conducts.

The H-bridge has a 10 kHz max rate, but that might be good enough. I\'ll see if I can get someone to make a measurement.

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

 

[Bill Sloman]

On Tuesday, October 20, 2020 at 1:32:11 PM UTC+11, Ricketty C wrote:

We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load, so a PWM drive at a high enough frequency would approximate a constant current to the motor. A smoothing cap at the source (we already have 2000 uF) helps that a lot. The H-bridge would be retained to allow the polarity to the motor to be reversed.

Is this a method used? It would lower the I2R heating in the motor with a lower continuous current rather than a higher pulsed current with a duty cycle.

We have scope plots showing a 500 Hz pulse rate is not fast enough to make that work. I need to ask if we can get some better measurements to see how long it takes for the motor current to drop significantly when the H-bridge removes the power connection and the reverse diode conducts.

The H-bridge has a 10 kHz max rate, but that might be good enough. I\'ll see if I can get someone to make a measurement.

Sloman A.W., Buggs P., Molloy J., and Stewart D. “A microcontroller-based driver to stabilise the temperature of an optical stage to 1mK in the range 4C to 38C, using a Peltier heat pump and a thermistor sensor” Measurement Science and Technology, 7 1653-64 (1996)

We used pulse-width-modulation with an H-bridge to deliver an essentially DC current - up to 3A - to drive the Peltier junction. The H-bridge switched no faster than 200kHz, and the pulse-width pattern repeated at 17.4kHz., but it was fudged so that a 50% duty cycle showed up as a 139Khz square wave, which made filtering out the low frequency components a lot easier. E-mail me if you want a copy of the paper.

You can certainly use an H-bridge to produce a waveform which works as a sine wave of lower peak voltage amplitude that the supply rail. I\'ve done it in other applications. You do have to worry about the high frequency components coming out of the H-brige, but you can filter them close to the H-bridge and stop them radiating form the motor leads and warming up motor.

--
Bill Sloman, Sydney

 

[bitrex]

On 10/19/2020 10:32 PM, Ricketty C wrote:

We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load, so a PWM drive at a high enough frequency would approximate a constant current to the motor. A smoothing cap at the source (we already have 2000 uF) helps that a lot. The H-bridge would be retained to allow the polarity to the motor to be reversed.

Is this a method used? It would lower the I2R heating in the motor with a lower continuous current rather than a higher pulsed current with a duty cycle.

We have scope plots showing a 500 Hz pulse rate is not fast enough to make that work. I need to ask if we can get some better measurements to see how long it takes for the motor current to drop significantly when the H-bridge removes the power connection and the reverse diode conducts.

The H-bridge has a 10 kHz max rate, but that might be good enough. I\'ll see if I can get someone to make a measurement.

This project again? What motor? Need three engineers to PWM drive a
motor? Don\'t understand. Here\'s Gordon Ramsay on electronic design:
<https://youtu.be/6Gp5AYEKETs?t=824>

 

[Jasen Betts]

On 2020-10-20, Ricketty C <gnuarm.deletethisbit@gmail.com> wrote:

We brought a couple of new electronic designers onto the ventilator
project I\'m working on. One of them was suggesting we control the
motor with a variable voltage rather than an H-bridge to switch the
current in a PWM manner. He mentioned some high power amps to drive
this, but they are class AB and so would have high losses at anything
other than the maximum output.

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor?

no, but if you\'re interested in controlloing the speed it is.

> The motor is an inherently inductive load,

but the innertia of the mechanincal load looks inherently capacitive.

> so a PWM drive at a high enough frequency would approximate a constant current to the motor.

Which will get you a constant torque.

A smoothing cap at the source (we already have 2000 uF) helps that a lot.

The H-bridge would be retained to allow the polarity to the motor to be reversed.

Is this a method used?
It would lower the I2R heating in the motor with a lower continuous current rather than a higher pulsed current with a duty cycle.

huh what method, cap where?

--
Jasen.

 

[Martin Brown]

On 20/10/2020 07:14, Jasen Betts wrote:

On 2020-10-20, Ricketty C <gnuarm.deletethisbit@gmail.com> wrote:
We brought a couple of new electronic designers onto the ventilator
project I\'m working on. One of them was suggesting we control the
motor with a variable voltage rather than an H-bridge to switch the
current in a PWM manner. He mentioned some high power amps to drive
this, but they are class AB and so would have high losses at anything
other than the maximum output.

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor?

no, but if you\'re interested in controlloing the speed it is.

Though closed loop and monitoring rpm against the target would be
better. Optical sensor and a mirror flat on the shaft.

The motor is an inherently inductive load,

but the innertia of the mechanincal load looks inherently capacitive.

so a PWM drive at a high enough frequency would approximate a constant current to the motor.

Which will get you a constant torque.

Which might be good enough if the mechanical load is fairly constant.
Or monitor the voltage at the motor and control the PWM that way.

Linear power amplifiers would tend to generate a lot of waste heat.

--
Regards,
Martin Brown

 

[dcaster@krl.org]

On Monday, October 19, 2020 at 10:32:11 PM UTC-4, Ricketty C wrote:

We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load, so a PWM drive at a high enough frequency would approximate a constant current to the motor. A smoothing cap at the source (we already have 2000 uF) helps that a lot. The H-bridge would be retained to allow the polarity to the motor to be reversed.

I suspect that you do not need a very constant current. Consider a AC/DC motor run on AC. Not constant current ,but the rpm is essentially constant. And does some variation in the motor speed make any difference for a ventilator?

Just suggesting you check the requirements.

Dan

Is this a method used? It would lower the I2R heating in the motor with a lower continuous current rather than a higher pulsed current with a duty cycle.

We have scope plots showing a 500 Hz pulse rate is not fast enough to make that work. I need to ask if we can get some better measurements to see how long it takes for the motor current to drop significantly when the H-bridge removes the power connection and the reverse diode conducts.

The H-bridge has a 10 kHz max rate, but that might be good enough. I\'ll see if I can get someone to make a measurement.

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

 

[Bill Sloman]

On Wednesday, October 21, 2020 at 12:54:06 AM UTC+11, dcaster@krl.org wrote:

On Monday, October 19, 2020 at 10:32:11 PM UTC-4, Ricketty C wrote:
We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load, so a PWM drive at a high enough frequency would approximate a constant current to the motor. A smoothing cap at the source (we already have 2000 uF) helps that a lot. The H-bridge would be retained to allow the polarity to the motor to be reversed.

I suspect that you do not need a very constant current. Consider a AC/DC motor run on AC. Not constant current ,but the rpm is essentially constant. And does some variation in the motor speed make any difference for a ventilator?

All electric motors are essentially synchronous motors and depend on getting sine waves of current through the windings. DC motors just use brushes and commutators to do the switching.

The frequency and phase of the sine waves have to line up with the rotation of the motor - in a DC motor the commutator does that for you. Getting synchronous motors spun up and spinning at the right speed can be tricky.

When you go away from sinewaves, you get more heating when generating the same torque. If you put lots of high-frequency current into the motor windings it doesn\'t do a thing to generate extra torque, but will heat up the pole pieces, which won\'t help their magnetic properties.

The question didn\'t suggest that Rick C. hard thought all that hard about what sort of current he need to feed into to the motor. He didn\'t actually identify the sort of motor he was using, which wasn\'t a good start.

--
Bill Sloman, Sydney

 

[Ricketty C]

On Tuesday, October 20, 2020 at 2:30:56 AM UTC-4, Jasen Betts wrote:

On 2020-10-20, Ricketty C <gnuarm.deletethisbit@gmail.com> wrote:
We brought a couple of new electronic designers onto the ventilator
project I\'m working on. One of them was suggesting we control the
motor with a variable voltage rather than an H-bridge to switch the
current in a PWM manner. He mentioned some high power amps to drive
this, but they are class AB and so would have high losses at anything
other than the maximum output.

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor?

no, but if you\'re interested in controlloing the speed it is.

The motor is an inherently inductive load,

but the innertia of the mechanincal load looks inherently capacitive.

so a PWM drive at a high enough frequency would approximate a constant current to the motor.

Which will get you a constant torque.

A smoothing cap at the source (we already have 2000 uF) helps that a lot.

The H-bridge would be retained to allow the polarity to the motor to be reversed.

Is this a method used?
It would lower the I2R heating in the motor with a lower continuous current rather than a higher pulsed current with a duty cycle.

huh what method, cap where?

\"At the source\"... the power source to the circuit. The H-bridge data sheet shows a large capacitor at this point, I believe to supply the peak current and also to mitigate the dI/dt voltage spike when the motor is stopped or reversed.

--

Rick C.

+ Get 1,000 miles of free Supercharging
+ Tesla referral code - https://ts.la/richard11209

 

[Ricketty C]

On Tuesday, October 20, 2020 at 9:54:06 AM UTC-4, dca...@krl.org wrote:

On Monday, October 19, 2020 at 10:32:11 PM UTC-4, Ricketty C wrote:
We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output..

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load, so a PWM drive at a high enough frequency would approximate a constant current to the motor. A smoothing cap at the source (we already have 2000 uF) helps that a lot. The H-bridge would be retained to allow the polarity to the motor to be reversed.

I suspect that you do not need a very constant current. Consider a AC/DC motor run on AC. Not constant current ,but the rpm is essentially constant. And does some variation in the motor speed make any difference for a ventilator?

Just suggesting you check the requirements.

The operation of the motor is controlled to produce the desired effect. I am talking about upping the PWM rate to approximate a constant current drive rather than the present pulsed drive we have now. The switching losses in the H-bridge will be higher, but the losses in the motor where we are having some trouble are lower. Drive with 3 amps continuously or drive with 6 amps at a 50/50 duty cycle. The 6 amp drive has much higher I²R losses in the motor, about 2x I believe.

--

Rick C.

-- Get 1,000 miles of free Supercharging
-- Tesla referral code - https://ts.la/richard11209

 

[server]

On Tue, 20 Oct 2020 06:14:32 -0000 (UTC), Jasen Betts
<usenet@revmaps.no-ip.org> wrote:

On 2020-10-20, Ricketty C <gnuarm.deletethisbit@gmail.com> wrote:
We brought a couple of new electronic designers onto the ventilator
project I\'m working on. One of them was suggesting we control the
motor with a variable voltage rather than an H-bridge to switch the
current in a PWM manner. He mentioned some high power amps to drive
this, but they are class AB and so would have high losses at anything
other than the maximum output.

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor?

no, but if you\'re interested in controlloing the speed it is.

The motor is an inherently inductive load,

but the innertia of the mechanincal load looks inherently capacitive.

so a PWM drive at a high enough frequency would approximate a constant current to the motor.

Which will get you a constant torque.

A smoothing cap at the source (we already have 2000 uF) helps that a lot.

The H-bridge would be retained to allow the polarity to the motor to be reversed.

Is this a method used?
It would lower the I2R heating in the motor with a lower continuous current rather than a higher pulsed current with a duty cycle.

huh what method, cap where?

He sounds confused about voltage and current and motors. Why not just
buy a motor driver box? It\'s less likely to kill people.

https://www.amazon.com/Generic-Pulse-Modulator-Control-Controller/dp/B00M6Q8WZ6

Or at least read some online tutorials first.



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[Michael Kellett]

On 20/10/2020 15:23, Ricketty C wrote:

On Tuesday, October 20, 2020 at 9:54:06 AM UTC-4, dca...@krl.org wrote:
On Monday, October 19, 2020 at 10:32:11 PM UTC-4, Ricketty C wrote:
We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load, so a PWM drive at a high enough frequency would approximate a constant current to the motor. A smoothing cap at the source (we already have 2000 uF) helps that a lot. The H-bridge would be retained to allow the polarity to the motor to be reversed.

I suspect that you do not need a very constant current. Consider a AC/DC motor run on AC. Not constant current ,but the rpm is essentially constant. And does some variation in the motor speed make any difference for a ventilator?

Just suggesting you check the requirements.

The operation of the motor is controlled to produce the desired effect. I am talking about upping the PWM rate to approximate a constant current drive rather than the present pulsed drive we have now. The switching losses in the H-bridge will be higher, but the losses in the motor where we are having some trouble are lower. Drive with 3 amps continuously or drive with 6 amps at a 50/50 duty cycle. The 6 amp drive has much higher I²R losses in the motor, about 2x I believe.

This might help a bit, this guy works for Portescap and they make nice
motors and generally seem to know what they are doing. (I\'m assuming it
is a brushed motor you want to control.)

https://www.machinedesign.com/materials/article/21125511/controlling-brushed-dc-motors-using-pwm

From what I know of your application you might need some kind of
feedback control of motor speed.

MK

 

[Lasse Langwadt Christensen]

tirsdag den 20. oktober 2020 kl. 16.23.45 UTC+2 skrev Ricketty C:

On Tuesday, October 20, 2020 at 9:54:06 AM UTC-4, dca...@krl.org wrote:
On Monday, October 19, 2020 at 10:32:11 PM UTC-4, Ricketty C wrote:
We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load, so a PWM drive at a high enough frequency would approximate a constant current to the motor. A smoothing cap at the source (we already have 2000 uF) helps that a lot. The H-bridge would be retained to allow the polarity to the motor to be reversed.

I suspect that you do not need a very constant current. Consider a AC/DC motor run on AC. Not constant current ,but the rpm is essentially constant. And does some variation in the motor speed make any difference for a ventilator?

Just suggesting you check the requirements.

The operation of the motor is controlled to produce the desired effect. I am talking about upping the PWM rate to approximate a constant current drive rather than the present pulsed drive we have now. The switching losses in the H-bridge will be higher, but the losses in the motor where we are having some trouble are lower. Drive with 3 amps continuously or drive with 6 amps at a 50/50 duty cycle. The 6 amp drive has much higher I²R losses in the motor, about 2x I believe.

http://jfsimon.net/public/Power_Semiconductor_Applications_Philips.pdf

page 285-

 

[Ricketty C]

On Tuesday, October 20, 2020 at 12:08:26 PM UTC-4, Michael Kellett wrote:

On 20/10/2020 15:23, Ricketty C wrote:
On Tuesday, October 20, 2020 at 9:54:06 AM UTC-4, dca...@krl.org wrote:
On Monday, October 19, 2020 at 10:32:11 PM UTC-4, Ricketty C wrote:
We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load, so a PWM drive at a high enough frequency would approximate a constant current to the motor. A smoothing cap at the source (we already have 2000 uF) helps that a lot. The H-bridge would be retained to allow the polarity to the motor to be reversed.

I suspect that you do not need a very constant current. Consider a AC/DC motor run on AC. Not constant current ,but the rpm is essentially constant. And does some variation in the motor speed make any difference for a ventilator?

Just suggesting you check the requirements.

The operation of the motor is controlled to produce the desired effect. I am talking about upping the PWM rate to approximate a constant current drive rather than the present pulsed drive we have now. The switching losses in the H-bridge will be higher, but the losses in the motor where we are having some trouble are lower. Drive with 3 amps continuously or drive with 6 amps at a 50/50 duty cycle. The 6 amp drive has much higher I²R losses in the motor, about 2x I believe.


This might help a bit, this guy works for Portescap and they make nice
motors and generally seem to know what they are doing. (I\'m assuming it
is a brushed motor you want to control.)

https://www.machinedesign.com/materials/article/21125511/controlling-brushed-dc-motors-using-pwm

From what I know of your application you might need some kind of
feedback control of motor speed.

MK

Thank you. Yes, we have a PID control loop. There are a number of issues in the thing being controlled starting with the motor itself that we don\'t have characterized. The guy doing most of the motor work is a mechanical engineer who seems to be happy with a hunt and peck approach to finding a workable solution. He has created a rather large number of hands to press on the bag, each time changing it without properly understanding how it might interact with the bag it is pressing on... resulting in yet another attempt needed. He\'s also the project leader so not much I can tell him.

We are making progress though and may have something ready for a manufacturer to look at by the end of the year. Not sure about the software. That\'s not really being worked on at the moment while the hardware is being banged on. Not sure why we can\'t come to terms on an interface spec and work on both in parallel. Too many members seem to have a hobbyist approach using the REPL method I guess, not that REPL is bad, just not the only approach.

I\'ve been trying to explain the nature of PWM control to these guys, but it isn\'t sinking in. Your link should help a lot. But my biggest problem is getting people to stop experimenting and start doing thoughtful, rational design.

Even when Larkin is doing his experiments, it\'s not just to find out if something will work or not. He has a plan to take measurements that will give him information. The garage mentality is the pits.

--

Rick C.

-+ Get 1,000 miles of free Supercharging
-+ Tesla referral code - https://ts.la/richard11209

 

[whit3rd]

On Monday, October 19, 2020 at 7:32:11 PM UTC-7, Ricketty C wrote:

We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load...

Motors have iron in the windings, and there\'s leakage inductance, BUT that\'s at audio and below. At
typical SMPS frequencies, that iron is resistive, and rotor inertia is capacitive...

Yes, switching is a way to supply a motor, but it\'s better to consider using a HF-capable choke in series,
rather than modeling the motor as an inductor, if you want to keep your design on-track. Why
does one want variable motor control, can\'t you just do ON/OFF?

 

[Ricketty C]

On Tuesday, October 20, 2020 at 2:44:25 PM UTC-4, whit3rd wrote:

On Monday, October 19, 2020 at 7:32:11 PM UTC-7, Ricketty C wrote:
We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load...

Motors have iron in the windings, and there\'s leakage inductance, BUT that\'s at audio and below. At
typical SMPS frequencies, that iron is resistive, and rotor inertia is capacitive...

Yes, switching is a way to supply a motor, but it\'s better to consider using a HF-capable choke in series,
rather than modeling the motor as an inductor, if you want to keep your design on-track. Why
does one want variable motor control, can\'t you just do ON/OFF?

The motor turns an arm with a \"hand\" that pushes on a bag that expels air that inflates a patient\'s lungs. The mode of operation is constant pressure.. There is a feedback system to maintain the pressure during the inspiratory portion of the cycle. None of that is the target of this matter. I\'m just looking to reduce the losses and noise of operating the motor.

It would be useful to find a H-bridge motor controller that is rated to work at PWM rates more in the 20 kHz range with 15A+ loads. The VNH3SP30-E we are planning to use has an upper limit of 10 kHz.

--

Rick C.

+- Get 1,000 miles of free Supercharging
+- Tesla referral code - https://ts.la/richard11209

 

[Martin Brown]

On 20/10/2020 20:16, Ricketty C wrote:

The motor turns an arm with a \"hand\" that pushes on a bag that expels
air that inflates a patient\'s lungs. The mode of operation is
constant pressure. There is a feedback system to maintain the
pressure during the inspiratory portion of the cycle. None of that
is the target of this matter. I\'m just looking to reduce the losses
and noise of operating the motor.

Why is the motor power requirement here so large?

You are trying to inflate and deflate the patients lungs slowly and
steadily with gentle periodic movements not pump up a car tyre quickly.

It should be possible to use a relatively modest motor and gearing to
convert that basic rotary motion into whatever movement is required.

It would be useful to find a H-bridge motor controller that is rated
to work at PWM rates more in the 20 kHz range with 15A+ loads. The
VNH3SP30-E we are planning to use has an upper limit of 10 kHz.

That is more the sort of motor power control kit I would expect to see
on electric scale model trains on 3.5\" or 5\" gauge. They can typically
pull three or four bogeys loaded with kids round on a track. Or at least
they could until Covid restrictions came into play. eg.

https://www.4qd.co.uk/product/dno/

RFI is something else to look out for motor power control in a hospital
setting. It caught out Crossrail in the UK badly as their signalling
system was confused by dynamic interference from the rolling stock motor
controllers. They are not the only ones Hitachi\'s Azuma trains didn\'t
get on with the signalling system north of York when first delivered.

https://www.railmagazine.com/news/network/signalling-interference-halts-azuma-tests-north-of-york

--
Regards,
Martin Brown

 

[Klaus Kragelund]

On Tuesday, October 20, 2020 at 4:15:08 PM UTC+2, Bill Sloman wrote:

On Wednesday, October 21, 2020 at 12:54:06 AM UTC+11, dca...@krl.org wrote:
On Monday, October 19, 2020 at 10:32:11 PM UTC-4, Ricketty C wrote:
We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output..

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load, so a PWM drive at a high enough frequency would approximate a constant current to the motor. A smoothing cap at the source (we already have 2000 uF) helps that a lot. The H-bridge would be retained to allow the polarity to the motor to be reversed.

I suspect that you do not need a very constant current. Consider a AC/DC motor run on AC. Not constant current ,but the rpm is essentially constant. And does some variation in the motor speed make any difference for a ventilator?
All electric motors are essentially synchronous motors and depend on getting sine waves of current through the windings. DC motors just use brushes and commutators to do the switching.

The frequency and phase of the sine waves have to line up with the rotation of the motor - in a DC motor the commutator does that for you. Getting synchronous motors spun up and spinning at the right speed can be tricky.

When you go away from sinewaves, you get more heating when generating the same torque. If you put lots of high-frequency current into the motor windings it doesn\'t do a thing to generate extra torque, but will heat up the pole pieces, which won\'t help their magnetic properties.

The question didn\'t suggest that Rick C. hard thought all that hard about what sort of current he need to feed into to the motor. He didn\'t actually identify the sort of motor he was using, which wasn\'t a good start.

He is asking the right question. The inductance of a motor winding is very high, and a 10kHz to 20kHz PWM is typical to control the motor winding current. it will not generate a lot of losses since the inductance reduces the ripple current to very low amplitude

Cheers

Klaus

 

[Klaus Kragelund]

On Tuesday, October 20, 2020 at 8:44:25 PM UTC+2, whit3rd wrote:

On Monday, October 19, 2020 at 7:32:11 PM UTC-7, Ricketty C wrote:
We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load...

Motors have iron in the windings, and there\'s leakage inductance, BUT that\'s at audio and below. At
typical SMPS frequencies, that iron is resistive, and rotor inertia is capacitive...

That is not correct. At switching frequency, the winding is inductive. Rotor inerita has almost no effect on the winding. If you have burried versus surface magnets, the winding inductance varies with respect to rotor orientation, but that is another issue, relevant for the motor control algoritm

Yes, switching is a way to supply a motor, but it\'s better to consider using a HF-capable choke in series,
rather than modeling the motor as an inductor, if you want to keep your design on-track.
Adding more components that is not needed

Why
does one want variable motor control, can\'t you just do ON/OFF?

 

[John Larkin]

On Tue, 20 Oct 2020 14:15:24 -0700 (PDT), Klaus Kragelund
<klauskvik@hotmail.com> wrote:

On Tuesday, October 20, 2020 at 8:44:25 PM UTC+2, whit3rd wrote:
On Monday, October 19, 2020 at 7:32:11 PM UTC-7, Ricketty C wrote:
We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load...

Motors have iron in the windings, and there\'s leakage inductance, BUT that\'s at audio and below. At
typical SMPS frequencies, that iron is resistive, and rotor inertia is capacitive...

That is not correct. At switching frequency, the winding is inductive. Rotor inerita has almost no effect on the winding. If you have burried versus surface magnets, the winding inductance varies with respect to rotor orientation, but that is another issue, relevant for the motor control algoritm

Rotor/load inertia gets interesting when you need deceleration. That
stored energy has to go somewhere. An H-bridge will happily extract
the mechanical energy from the system and stuff it back into the power
supply.

 

[Klaus Kragelund]

On Tuesday, October 20, 2020 at 11:43:40 PM UTC+2, John Larkin wrote:

On Tue, 20 Oct 2020 14:15:24 -0700 (PDT), Klaus Kragelund
klau...@hotmail.com> wrote:

On Tuesday, October 20, 2020 at 8:44:25 PM UTC+2, whit3rd wrote:
On Monday, October 19, 2020 at 7:32:11 PM UTC-7, Ricketty C wrote:
We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.

Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load...

Motors have iron in the windings, and there\'s leakage inductance, BUT that\'s at audio and below. At
typical SMPS frequencies, that iron is resistive, and rotor inertia is capacitive...

That is not correct. At switching frequency, the winding is inductive. Rotor inerita has almost no effect on the winding. If you have burried versus surface magnets, the winding inductance varies with respect to rotor orientation, but that is another issue, relevant for the motor control algoritm
Rotor/load inertia gets interesting when you need deceleration. That
stored energy has to go somewhere. An H-bridge will happily extract
the mechanical energy from the system and stuff it back into the power
supply.

Correct, but that is the same case if he used a H bridge with PWM, or a converter in front

Cheers

Klaus